1. Field of the Invention
The present invention relates to a method and device for removing the photoresist on the wafer edge, particularly to a method, optics and an auto-focusing system for removing the photoresist on the wafer edge more accurately by using wafer edge exposure.
2. Description of the Related Art
Smaller and smaller characteristic dimensions are desired by the rapidly developed micro-electronics industry, and the shrinking of characteristic dimensions relies upon photolithography processes to a great extent.
A photolithography process mainly comprises the following steps: coating with a photoresist, exposure and development of the photoresist, wherein as the first step of the photolithography process, the quality of coating of a photoresist has a direct impact on the yield of subsequent processes. With respect to photoresist coating, it is necessary for the layer of a photoresist coated on a wafer to have uniform thickness, which is generally realized by spin coating in the prior art. Spin coating is a procedure, in which a wafer is fixed on the rotating shaft of a spin coating stage and is coated with a photoresist as it is rotated, uniformly spreading the photoresist on the surface of the wafer by centrifugal force. Although the photoresist layer formed by spin coating may have uniform thickness over the central area (also called “effective area”) of the wafer, photoresist accumulation tends to occur at the edge area of the wafer, which is called edge bead. Photoresist accumulated on a wafer edge is liable to chip off and thereby causes particles in subsequent processes (e.g., baking), affecting the pattern transfer of the central area, and preventing formation of good patterns. Furthermore, photoresist accumulation may incur problems such as stage contamination. Hence, an edge bead removal process is generally employed to remove the photoresist on the wafer edge after the photoresist coating.
There are two kinds of common processes for edge bead removal, one is chemical method in which an amount of a solvent is dispensed on both sides of the wafer and is so controlled as not to reach the central area of the wafer. However, edge bead removal by this chemical method results in a relatively rough edge, which has a low spatial resolution of about 0.5-1 mm. Another method is optical method, that is, wafer edge exposure (WEE), in which after the photoresist coating, a pattern is transferred onto the photoresist on the wafer edge by the projection of a pattern defining element (such as, a mask with an aperture) on the wafer edge, as shown in FIG. 1. After the pattern exposure on the wafer edge is completed, the photoresist on the wafer edge is removed by a development process. As compared to the chemical method, the width and position of edge bead to be removed can be controlled more precisely, and improved spatial resolution can be realized by the optical method. However, since the projection of the pattern defining element may lead to a blurring edge of about 50 μm, a transition region of a considerable width where photoresist is partially removed still retains after the edge bead removal by the optical method. Such a transition region is liable to introduce defects, reducing yield of the process consequently.
Therefore, it is desired to provide a method and device capable of removing photoresist on a wafer edge more precisely.